1. Field of the Invention
The present invention relates to the detection of low concentrations of dissolved materials in solvents and, particularly, to optical analysis procedures which may be performed in a continuous manner. More specifically, this invention is directed to improvements in sensors comprising hollow optical waveguides and, especially, to liquid core waveguide sensors with enhancements in the means for delivery of analysis/excitation light to the core liquid. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
It is known to analyze fluids in cuvettes formed from optically transparent material, usually quartz, which have been drawn into the form of cylindrical, thin-walled capillaries. The fluid to be analyzed is confined in the capillary, i.e., forms a liquid core. Materials dissolved in such a confined liquid core can, even at low concentrations, be detected due to their characteristic optical absorbance of analysis light, their characteristic fluorescence when excited by analysis light of the appropriate wavelength or their Raman spectra. The coupling of light into and out of a capillary employed in such prior art analysis procedures has usually been accomplished at the opposite end faces of the capillary with the analysis light being directed through the cuvette axially thereof. The solvent with dissolved compounds is customarily delivered to and exhausted from the cuvette via radially oriented liquid channels.
It is also known, in the interest of limiting light leaving a cuvette in directions transverse to the axis thereof, to employ a coating on the inner wall of the capillary. When such a coating has a refractive index which is lower than that of the commonly used solvents in the visible and ultraviolet wavelength spectra, total light reflection will occur at the liquid/coating interface. This reflection decreases light loss in the liquid core and longer optical path lengths, which increase the sensitivity of a sensor cell comprising such a liquid core waveguide, are thus possible. The coating on the inner wall of the capillary, where the core liquid is to be an aqueous solution, will preferably consist of an amorphous fluorinated polymer, such as Teflon AF 1600 or Teflon AF 2400. These coating materials respectively have refractive indices of 1.31 and 1.29 in the wavelength region of the sodium-D-line.
The ultraviolet, visible and infrared regions of the light spectrum have long been used in spectroscopy for liquid and gas analysis. Commonly used analysis methods are transmittance and, as noted above, absorbance and fluorescence. To accomplish the desired measurements employing these methods, liquid filled cuvettes are positioned in the path of analysis light generated by an appropriate source. In addition, as disclosed in U.S. Pat. No. 4,260,257, rotational symmetric liquid-flushed, flow-through cells, with small diameters to reduce the sample volume, have been proposed for liquid and gas analysis.
Fiber optics have been developed and improved in recent years, and fiber optic coupling between light sources and cuvettes and flow-through cells has been accomplished. Thus, light from a source may be efficiently coupled into a fiber optic element, consisting of either a single fiber or a fiber bundle, and transmitted thereby into or away from a cuvette. The fibers employed for such purposes preferably consist of glass or quartz, depending on the wavelength of the light to be guided. Light emitted from the end of a fiber optic element, within the angle dictated by the numerical aperture thereof, may be converted by a micro-lens into parallel light which is guided through a cuvette. After such light passes through the relatively short optical path length of a standard cuvette, typically 10 cm, the light is coupled, by means of a focusing lens, into a fiber optic element connected to a light detector. The light detector may be a wavelength-selective system, e.g., a polychromator, or a detector, e.g., a silicon photo-diode, or a photomultiplier tube which may be provided with an analyte-adapted narrow or broad-band optical filter.
In the interest of signal enhancement, quartz and glass tubes with small outer diameters and thin walls, so-called capillaries, have been used as flow-through cells. The choice of quartz and glass tubes, in part, has been based upon the fact that such tubes are chemically inert against many liquids and solvents. These capillaries can be provided with an internal, and possibly also an external, metal coating in an environment where there would be negligible corrosion. However, metal-coated capillaries have the disadvantage of high light loss. Thus, the optical path length is relatively small for a pre-defined signal-to-noise ratio.
As noted above, by providing an optically transparent capillary with a coating of a suitable amorphous fluorinated polymer, an efficient liquid core optical waveguide may be produced. On this point, the following references may be taken into consideration: xe2x80x9cOptical Characteristics of Teflon AF Fluoroplastic Materialsxe2x80x9d, by J. H. Lowry et al, Optical Engineering, Volume 31, page 1982 (1992); U.S. Pat. No. 5,184,192; xe2x80x9cRaising the Sensitivity Benchmark in Diode Array Detection with Optical Improvementsxe2x80x9d, by P. DeLand, Internat. Laboratory 12C-12H (July 1998); xe2x80x9cA Cylindrical Liquid Core Waveguidexe2x80x9d, by P. Dress et al, Appl. Phys. B63, page 12 (1996) and U.S. Pat. No. 5,570,447. By insuring that the liquid core has a higher refractive index than a coaxial layer of material, as discussed above, coupled light is mainly guided in the liquid core because of the total reflection which occurs at, for example, the liquid/coating interface. In other words, optical losses resulting from transverse emission through the wall of the capillary are substantially eliminated through the use of low refractive index amorphous fluorinated polymers. Liquid core waveguides of the type described, wherein the low refractive index polymer forms either an interior or an exterior coating on the capillary, or forms the capillary itself, can be used as absorption sensors with high resolution. In such absorption sensors, light is axially coupled into and out of the capillary at the end face(s). Such light coupling has typically been accomplished by utilizing some type of focusing device between the light source and the capillary.
The problems concerned with transmission of light at wavelengths below 250 nm has been discussed in the article xe2x80x9cUV-stabilized silica based fiber for applications around 200 nm wavelengthxe2x80x9d, by K.-F. Klein et al., Sensors and Actuators B, Vol. 39-123, 305-309 (1997). This article suggests that optical fibers capable of stable transmission of light with wavelengths below 250 nm would allow the field of fiber optic applications to be significantly expanded. For example, field-usable sensors for water pollution by detection of nitrate, nitrite and residual chlorine, which have strong absorption bands below 250 nm, have long been needed.
The principals of capillary-like coated liquid core waveguides have been surveyed in the articled entitled xe2x80x9cCapillary Waveguide Sensorsxe2x80x9d, by O. S. Wolfbeis, Anal. Chem, Vol. 15, page 225 (1996). Side illumination of optical waveguides has also been previously suggested. The sensoric properties of such a side illuminated light guide are based on chemical or physical changes in its specific inner coating by the influence of the detectible substance. These changes include the fluorescence excitation of the inner coating by detectible substances. In such case, advantage is taken of the fact that the magnitude and spectral composition of a fluorescence signal changes with the detectible substances diffusing in and out of the coating. In such a sensor, the generated fluorescent light would be detected at the end of a short capillary.
A disadvantage of the conventional technique of axial coupling of light into a liquid core optical waveguide resides in the fact that the lowest detectible concentration of a substance dissolved in a liquid scales within the effective optical path length of the light to be collected and analyzed in the liquid. An additional disadvantage of axial light in-coupling resides in the fact that, if the optical characteristic to be measured is fluorescence, it becomes difficult to distinguish between the fluorescent light of interest as generated in the core and the analysis/excitation traveling through the core light since both types of light are guided to the end(s) of the waveguide and, accordingly, are simultaneously incident on the collector.
The present invention provides a sensor, based upon a liquid core optical waveguide, which increases the detection limit, of dissolved substances in fluids at acceptable apparatus dimensions. A sensor in accordance with the invention permits on-line analysis whereby xe2x80x9ccontinuousxe2x80x9d detection of a substance in a flow can be accomplished. The invention also encompasses a novel and improved method of trace analysis which employs a hollow optical waveguide. The invention further encompasses the use, particularly for fluorescence applications, of analysis/excitation light with wavelengths less than 230 nm.
In accordance with a preferred embodiment of the invention, trace analysis by measuring fluorescence is accomplished by transversely coupling analysis/excitation light into the liquid core of a hollow optical waveguide having an amorphous fluorinated polymer/core liquid interface. When operated in the fluorescence mode, for water analysis in the wavelength region below 250 nm, for example, an inner coating of Teflon AF on a transparent capillary yields both a very high transmission of analysis light into the waveguide and total reflection of a significant amount of the excited fluorescence light. The performance of the preferred embodiment of the invention can be enhanced, and the problem of absorption in the fiber optic elements used for excitation light input and light output may be overcome, by the employment of UV-stabilized fused silica fibers, especially hydrogen doped fused silica fibers, for coupling excitation light from the source into the waveguide and/or for collection of emitted fluorescence.
Applicants have also found that, in the case of a flow-through cell, sensitivity is maximized if the flow is as laminar as possible in the measurement region thereby avoiding disturbance of the total reflection at the interface between the flowing liquid core and the capillary. Such laminar flow in the measurement region can be realized by maintaining appropriate distances between the opposite ends of the measurement zone, i.e., the areas where light is coupled into and out of the cell, and the channels or conduits through which the core liquid flows into and out of the capillary. Restated, in accordance with the invention, the length of the measurement zone is less than that of the liquid flow path within the waveguide.
Applicants have also discovered that transverse coupling of excitation light, in combination with the light guiding property of the combination of a core liquid with a relatively high optical refractive index and a coaxial lower refractive index capillary, provides a three dimensional separation between in-coupled excitation light and the generated fluorescence light because the excitation light is not directed into the waveguide with an angle which is within the guiding properties of the waveguide. This three dimensional separation permits employment of a stretched, cylindrical waveguide wherein the emitted fluorescence light is guided inside the waveguide and can be detected at both end faces of the capillary.
An advantageous increase in the sensitivity of fluorescence sensors in accordance with the invention results from an increase of the total excitation light flux coupled into the liquid waveguide core region, i.e., increased excitation light intensity causes an increase of the generated fluorescence signal. To increase the excitation light input, in one embodiment of the invention, a line-formed array of fiber optic elements is employed with the individual optical fibers being oriented generally radially with respect to the liquid core waveguide and the array extending longitudinally with respect to the waveguide.
In order to decrease coupling losses at the interface between an individual fiber optic element employed for excitation light transmission and the exterior of the waveguide, a waveguide sensor in accordance with the invention will preferably be modified to present a flat surface which is complementary to the end of the fiber optic element. The flat surface on the waveguide may be provided by grinding the exterior of a cylindrical capillary or by fabricating the capillary as a rectangular tube, i.e., a quadratic liquid core waveguide is employed. In the case of grinding flats on the exterior of the cylindrical waveguide, anti-tangential grinding will increase the optical path length/absorption length of the excitation light by causing at least some of such light to undergo multiple reflections within the waveguide.
A hollow optical waveguide sensor in accordance with the invention may, in the interest of avoiding any deleterious effects of stray light, be surrounded by a reflective metal coating except in the areas where excitation light is to be transversely coupled into the waveguide.
In accordance with a further embodiment of the invention, a cylindrical light source oriented parallel with respect to the axis of a liquid core waveguide is utilized and the light provided by such source is focused by cylindrical lenses into the core of the waveguide.
In accordance with yet another embodiment of the invention, the waveguide comprises a thin, flexible coating, with a refractive index lower than that of the intended core liquid, is employed on a flexible capillary. Alternatively, a flexible waveguide comprised entirely of an amorphous fluorinated polymer, such as Teflon AF, may be utilized. In either case, the flexible waveguide is wrapped around a bar-shaped light source. Such an arrangement is particularly useful for excitation with light in the ultraviolet region.
The present invention is particularly well suited for on-line analysis, particularly a xe2x80x9ccontinuousxe2x80x9d measurement process, wherein fluorescence is the optical characteristic of interest. In the past, on-line analysis employing fluorescence measurements has been difficult. The present invention reduces or removes sources of possible reduction of the signal of interest through use of pumps, valves and, if necessary, storage containers. In the case of on-line or continuous analysis, the present invention contemplates selectively and temporarily separating the liquid core waveguide hydraulically from the remaining circulation to enable the excited core liquid to remain in place for a sufficient time for measurement of emitted fluorescent light or sufficient time for shifted absorption of different states of the substance of interest.
To summarize the advantages of the present invention, the sensitivity and detection limit of a sensor employing a liquid core optical waveguide is increased vis-a-vis the prior art. The apparatus and methods of the invention enable the on-line analysis of liquids by the employment of micro-pumps, valves and storage tanks. In the fluorescence measurement mode, a three-dimensional separation of transverse coupled excitation light and the stimulated fluorescent light is achieved and the fluorescent light to be analyzed may be collected at both end faces of the liquid core waveguide.
The present invention, through analysis of fluorescence produced as a result of UV excitation, permits the use of liquid core waveguides with measurement or optical path lengths in excess of 20 m. The foregoing is possible because the self-absorption of the fluorescent light by shifting to longer wavelengths can be neglected for low analyte concentrations. Also, through the use of specially doped UV stabilized fiber optic elements, particularly hydrogen doped elements, in combination with a coaxial Teflon AF layer on a hollow optical waveguide, fluorescence excitation with light at wavelengths below 250 nm, and particulary as short as 200 nm, is possible.
Through the employment of flexible liquid core waveguides, particularly waveguides comprised of Teflon AF tubing, sensor cells which may be caused to conform to the shape of light sources of various forms, particularly bar-formed lights, are possible. The possibility of using various light sources in the commonly found bar-formed shape permits excitation light to be coupled wavelength selectively into the waveguide, via a linear array of fiber optic elements arranged along the longitudinal axis of the waveguide, for example, and the stimulated fluorescence light can be detected and simultaneously resolved into its spectral components with, again by way of example, a polychromator.
In the practice of the present invention, in either the absorption or fluorescence mode, by employing fiber optic elements with a low water content in combination with core materials having a low water content, the measurement range can be extended to the visible and near infrared region of the light spectrum up to wavelengths of 2.3 xcexcm. Further, flexible optical fibers can be employed for the fiber optic excitation light transmission and/or fluorescent light collection elements. For example, it is possible to employ fibers comprised of silicate glasses from 380 to 1100 nm, fibers made of PMMA (acrylic) from 350 to 750 nm and fibers comprised of fluorinated polymers from 400 to 1600 nm.